The brightness of illumination sources is typically limited by the brightness of the light source used. For example, an illumination source that uses light emitting diodes (LEDs) typically has a brightness, measured in power per unit area per unit solid angle, the same as or less than that of the LEDs. The optics that collect the light from the LEDs will, at best, conserve the étendue of the LED source.
In some applications of illumination sources, such as projector illumination, illumination by LEDs is not a competitive option because the brightness of the LEDs that are currently available is too low. This is particularly a problem for the generation of green illumination light, a region of the visible spectrum where the semiconductor materials used in LEDs are less efficient at generating light.
Other types of light sources may be able to produce a sufficiently bright beam of light but also suffer from other drawbacks. For example, a high-pressure mercury lamp is typically able to provide sufficient light for a projection system, but this type of lamp is relatively inefficient, requires a high voltage supply, contains toxic mercury, and has a limited lifetime. Solid-state sources, such as LEDs are more efficient, operate at lower voltages, typically contain no mercury, and are therefore safer, and have longer lifetimes than lamps, often extending to several tens of thousands of hours.